feat(maestro): make web UI an installable PWA

Add a web app manifest, icons, and a runtime-caching service worker so the
UI can be installed on Android Chrome (and added to the iOS home screen).

- web/public/manifest.webmanifest: standalone display, theme/bg colors, icon set
- web/public/icon.svg + generated icon-192/512 and apple-touch-icon PNGs
- web/public/sw.js: caches the static shell for offline launch; never touches
  /api (HTTP or WS), so live-daemon behavior is unchanged
- index.html: link manifest + apple-touch-icon, iOS standalone metas
- main.ts: register the service worker (production only)

Assets land in web/dist via Vite's public/ dir and are embedded by the
existing include_dir! in maestro-web, so no Rust changes are needed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/public/icon.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <rect width="512" height="512" fill="#1a1d24" />
+  <circle cx="256" cy="256" r="70" fill="#5fe3a1" />
+  <circle cx="256" cy="256" r="120" fill="none" stroke="#5fe3a1" stroke-opacity="0.5" stroke-width="12" />
+  <circle cx="256" cy="256" r="170" fill="none" stroke="#5fe3a1" stroke-opacity="0.25" stroke-width="10" />
+</svg>